Method and apparatus for managing network elements in a satellite navigation data distribution system

ABSTRACT

Method and apparatus for managing a network element in a satellite navigation data distribution system is described. In one example, a network element includes a processor for processing satellite navigation data. For example, a network element may be a reference station, a hub, or a server in the satellite navigation data distribution system. The network element includes a memory for maintaining status variables associated with the processing of the satellite navigation data. The status variables may relate to the integrity of the satellite navigation data. The network element further includes a management agent for monitoring states of the status variables and communicating with a network management system to exchange information related to the states of the status variables. In one example, the management agent is configured to communicate using a simple network management protocol (SNMP).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 10/719,890, filed Nov. 21, 2003, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to satellite position locationsystems and, more particularly, to a method and apparatus for managingnetwork elements in a satellite navigation data distribution system.

2. Description of the Related Art

A satellite signal receiver for the Global Positioning System (GPS) usesmeasurements from several satellites to compute a position. The processof acquiring the GPS radio signal is enhanced in speed and sensitivityif the GPS receiver has prior access to a model of the satellite orbitand clock. This model is broadcast by the GPS satellites and is known asthe satellite navigation message. Once the GPS radio signal has beenacquired, the process of computing position requires the use ofinformation contained within the satellite navigation message.

The GPS satellite navigation message is transmitted in 1500-bit framesat 50 bits per second, as defined by ICD-GPS-200C. Thus, each frame istransmitted in 30 seconds. The 1500-bit frame of each broadcast includesfive sub-frames of 300 bits length. The first three sub-frames (Le., thefirst 900 bits) include the ephemeris information associated with theparticular broadcasting satellite. The ephemeris information containsprecise satellite orbit and time model information for a particularsatellite. The first three sub-frames are identically repeated in each1500-bit frame for a particular duration. The broadcast ephemerisinformation is typically valid for two to four hours into the future(from the time of broadcast) and is periodically updated by a satellitecontrol station. The fourth and fifth sub-frames contain part of asatellite almanac, which includes coarse ephemeris and time modelinformation for the entire satellite constellation. The contents of thefourth and fifth sub-frames change until the entire almanac istransmitted. The repetition period of the fourth and fifth sub-frames is12.5 minutes (i.e., the entire satellite almanac is contained in 15,000bits).

It is always slow (no faster than 18 seconds), frequently difficult, andsometimes impossible (in environments with very low signal strengths),for a GPS receiver to download ephemeris information from a satellite.For these reasons, it has long been known that it is advantageous tosend the ephemeris to a GPS receiver by some other means in lieu ofawaiting the transmission from the satellite. U.S. Pat. No. 4,445,118,issued Apr. 24, 1984, describes a technique that collects ephemerisinformation at a GPS reference station, and transmits aiding data to theremote GPS receiver via a wireless transmission. This technique ofproviding aiding data to a GPS receiver has become known as“Assisted-GPS”.

Presently, A-GPS reference stations receive ephemeris data for in-viewsatellites and store the entire ephemeris model (e.g., 900 bits) as adata file for distribution. The data file containing the ephemeris istransmitted to the remote receiver at some time after the initialcollection of the data (e.g., minutes later). This latency betweencollection and distribution of the ephemeris data may deleteriouslyaffect operation of the remote receiver. For example, the ephemeris datain use by the remote receiver may become invalid due to an unhealthysatellite. The remote receiver, however, will continue to use theinvalid ephemeris data for several minutes before receiving updatedephemeris data from the server.

Therefore, there exists a need in the art for a method and apparatusthat distributes satellite navigation data to a remote receiver withdecreased latency. In addition, there exists a need in the art for amethod and apparatus that monitors the integrity of the collectedsatellite navigation data, as well as the integrity of the networkelements distributing such satellite navigation data.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome by a methodand apparatus for distributing satellite navigation data. In oneembodiment, satellite signals are processed at each of a plurality ofreference stations to receive a respective plurality of satellitenavigation data streams. Packets are formed in response to saidplurality of satellite navigation data streams to generate a pluralityof packetized satellite navigation data streams. The packetizedsatellite navigation data streams are sent to a processing system. Theprocessing system removes duplicate packets within said plurality ofpacketized satellite navigation data streams to generate a combinedpacket stream. The combined packet stream is then sent into acommunication network.

Another aspect of the invention relates to a method and apparatus formanaging a network element in a satellite navigation data distributionsystem. In one embodiment, a network element includes a processor forprocessing satellite navigation data. For example, a network element maybe a reference station, a hub, or a server in the satellite navigationdata distribution system. The network element includes a memory formaintaining status variables associated with the processing of thesatellite navigation data. The status variables may relate to theintegrity of the satellite navigation data. The network element furtherincludes a management agent for monitoring states of the statusvariables and communicating with a network management system to exchangeinformation related to the states of the status variables. In oneembodiment, the management agent is configured to communicate using asimple network management protocol (SNMP).

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram depicting an exemplary embodiment of asatellite navigation data distribution system;

FIG. 2 is a data flow diagram depicting an exemplary embodiment of aprocess for distributing satellite navigation data from a referencestation to a server;

FIG. 3 is a flow diagram depicting an exemplary embodiment of a processfor decoding satellite signals to recover satellite navigation datawithin a reference station;

FIG. 4 is a flow diagram depicting an exemplary embodiment of a processfor concentrating satellite navigation data within a hub;

FIG. 5 is a flow diagram depicting an exemplary embodiment of a processfor decoding satellite navigation data at a server;

FIG. 6 is a block diagram depicting an exemplary embodiment of acomputer for implementing the processes and methods described herein;

FIG. 7 is a block diagram depicting another exemplary embodiment of asatellite navigation data distribution system constructed in accordancewith the invention;

FIG. 8 is a data flow diagram depicting an exemplary embodiment of amethod for managing a network element in the satellite navigation datadistribution system of FIG. 7 in accordance with the invention; and

FIG. 9 is a data flow diagram depicting another exemplary embodiment ofa method for managing a network element in the satellite navigation datadistribution system of FIG. 7 in accordance with the invention.

To facilitate understanding, identical reference numerals have beenused, wherever possible, to designate identical elements that are commonto the figures.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram depicting an exemplary embodiment of asatellite navigation data distribution system 100. The system 100comprises a plurality of reference stations 102 ₁ through 102 _(N)(collectively referred to as reference stations 102), a hub 108, and aserver 116. The reference stations 102 receive satellite navigation datafrom a plurality of satellites 105. The hub 108 collects the satellitenavigation data from the reference stations 102 and provides thesatellite navigation data to the server 116. The server 116 processesthe satellite navigation data to decode the various parameters definedtherein. The server 116 may then transmit information extracted from thesatellite navigation data to a requester 120.

More specifically, each of the reference stations 102 ₁ through 102 _(N)includes a respective one of GPS receivers 104 ₁ through 104 _(N)(collectively referred to as satellite signal receivers 104) forreceiving signals from satellites of the plurality of satellites 105that are in-view. Each of the GPS receivers 104 decodes the receivedsatellite signals to obtain satellite navigation data associated withthe in-view satellites. The satellite navigation data comprisessatellite navigation messages that are formatted into frames andsub-frames, as described above. The GPS receivers 104 are capable ofstreaming raw satellite navigation messages in real time. For example,certain NovAtel GPS receivers have this capability.

The reference stations 102 format the satellite navigation data streamsproduced by the GPS receivers 104 for transmission over thecommunications network 106 to the hub 108. In one embodiment, thereference stations 102 process the data streams to form packet streamscomprising internet protocol (IP) packets, which may be transmitted overthe communication link 106 using the uniform datagram protocol (UDP).The hub 108 processes the formatted data streams from the referencestations 102 (“reference station data streams”) to remove redundantinformation. The hub 108 produces a formatted data stream comprising theunique information from the reference station data streams satellitenavigation data from the reference stations 102 (“hub data stream”). Thehub 108 transmits the hub data stream to the server 116 using acommunication network 112. In one embodiment of the invention, one ormore additional hubs (“hub(s) 110”) are used to provide redundancy. Thehub(s) 110 operate in the same manner as the hub 108. Each of thecommunication networks 106 and 112 may comprise any type of networkknown in the art, such frame relay, asynchronous transfer mode (ATM)networks, and the like. Although the communication networks 106 and 112have been shown as separate networks, those skilled in the art willappreciate that networks 106 and 112 may comprise a single network.

In one embodiment, another reference station 114 may be disposed inproximity to the server 116. The reference station 114 includes a GPSreceiver 115 similar to the GPS receivers 104, and provides a formatteddata stream similar to those provided by the reference stations 102(“co-located reference station data stream”). The server 116 processesthe hub data stream(s) and the co-located reference station data stream,if available, to extract various parameters therefrom. For example, theserver 116 may extract one or more of ephemeris data, almanac data,ionosphere data, universal time (UTC) offset data, satellite healthdata, as well as the raw data bits comprising the satellite navigationmessages. Similar to the hubs 108 and 110, the server 116 may firstprocess the hub data stream(s) and the co-located reference station datastreams to remove redundant information. The extracted information maybe provided to the requester 120 using a communication network 118. Thecommunication network 118 may comprise a wireless communication networkor other type of communication network, such as the Internet.

FIG. 2 is a data flow diagram depicting an exemplary embodiment of aprocess 200 for distributing satellite navigation data from a referencestation to a server. The process 200 begins with a satellite navigationdata stream 202. The satellite navigation data stream 202 comprisessub-frames of satellite navigation messages broadcast by in-viewsatellites. The satellite navigation data stream 202 is provided asinput to a packetizer 204. The packetizer 204 formats the satellitenavigation data stream 202 into a packet stream 206. In one embodimentof the invention, each packet in the packet stream 206 includes asub-frame of the satellite navigation data stream 202. In addition, eachpacket in the packet stream 206 includes a header for identifying thesub-frame carried therein. For example, the header may include asatellite identifier and a time-of-week (TOW) value that uniquelyidentifies the associated sub-frame. The packet stream 206 may bedirectly output as a reference station data stream 208.

The reference station data stream 208 is provided as input to aconcentrator 210. The concentrator 210 also receives reference stationdata streams from other reference stations. The concentrator 210processes the reference station data streams to remove packets carryingredundant information. For example, two of the reference stations may bepositioned on the surface of the Earth so as to receive the satellitenavigation message from the same satellite. The reference station datastreams corresponding to these two reference stations will includepackets that define identical sub-frames. The redundant sub-frame is notnecessary and may be removed. The concentrator 210 provides a hub datastream 212 as output. The hub data stream 212 comprises a packet streamhaving unique information from the reference stations. For example, thehub data stream 212 may comprise a stream of packets carrying uniquesub-frames.

The hub data stream 212 is provided as input to a concentrator 214. Theconcentrator 214 may also receive additional hub data stream(s), as wellas an additional reference station data stream from a reference stationco-located with the server. The concentrator 214 operates in a similarmanner to the concentrator 210 to generate a server data stream 216. Theserver data stream 216 comprises a packet stream having uniqueinformation from the hubs and the co-located reference station. Theserver data stream 216 is provided as input to a decoder 218. Thedecoder 218 processes the server data stream 216 to extract satellitedata 220. The satellite data 220 comprises one or more of ephemeris,almanac, ionosphere data, UTC offset, satellite health status, and rawdata bits. The satellite data 220 is stored within a cache 222.

In one embodiment of the invention, a reference station may receive areference station data stream from another reference station. Thus, thepacket stream 206 within the reference station may be provided as inputto an optional concentrator 224. The concentrator 224 operations in asimilar manner to the concentrators 210 and 214 to remove redundantinformation and provide a unique reference station data stream 208 tothe hub.

FIG. 3 is a flow diagram depicting an exemplary embodiment of a process300 for decoding satellite signals to recover satellite navigation datawithin a reference station. The process 300 begins at step 302, wheresatellite navigation messages are received for a plurality of in-viewsatellites. At step 304, the sub-frames of the satellite navigationmessages are packetized to generate a packet stream (e.g., a stream ofIP packets). At step 306, a header is added to each packet within thepacket stream having a satellite identifier and a TOW value associatedwith a respective sub-frame. At optional step 308, the packet stream ismerged with packet stream(s) from other reference stations and packetscarrying redundant sub-frames are removed (e.g., packets having a headerwith the same satellite identifier and same TOW value). At step 310, thepacket stream is transmitted to a hub. For example, the packet streammay be transmitted using UDP.

FIG. 4 is a flow diagram depicting an exemplary embodiment of a process400 for concentrating satellite navigation data within a hub. Theprocess 400 begins at step 402, where packet streams are received from aplurality of reference stations. At step 404, packets of the packetstreams are analyzed to remove those packets carrying redundantinformation and merged to produce a hub packet stream. For example, theheaders of the packets may be analyzed for identify those headers havingthe same satellite identifier and the same TOW value. At step 406, thehub packet stream is transmitted to a server. For example, the hubpacket stream may be transmitted using UDP.

FIG. 5 is a flow diagram depicting an exemplary embodiment of a process500 for decoding satellite navigation data at a server. The process 500begins at step 502, where one or more hub data stream(s) are receivedfrom one or more hubs. At optional step 504, a reference station datastream is received from a reference station co-located with the server.At step 506, packets of the hub data stream(s) and the optionalreference station data are merged to produce a server data stream andpackets carrying redundant sub-frames are removed (e.g., packets havinga header with the same satellite identifier and same TOW value). At step508, the satellite navigation data carried by the server data stream isdecoded to produce satellite data. At step 510, the satellite data isstored within the server for transmission to a requester.

FIG. 6 is a block diagram depicting an exemplary embodiment of acomputer 600 suitable for implementing processes and methods describedabove. The computer 600 includes a central processing unit (CPU) 601, amemory 603, various support circuits 604, and an I/O interface 602. TheCPU 601 may be any type of microprocessor known in the art. The supportcircuits 604 for CPU 602 include conventional cache, power supplies,clock circuits, data registers, I/O interfaces, and the like. The I/Ointerface 602 may be directly coupled to the memory 603 or coupledthrough the CPU 601. The I/O interface 602 may be coupled to variousinput devices 612 and output devices 611, such as a conventionalkeyboard, mouse, printer, display, and the like.

The memory 603 may store all or portions of one or more programs and/ordata to implement the processes and methods described above. Althoughthe invention is disclosed as being implemented as a computer executinga software program, those skilled in the art will appreciate that theinvention may be implemented in hardware, software, or a combination ofhardware and software. Such implementations may include a number ofprocessors independently executing various programs and dedicatedhardware, such as application specific integrated circuits (ASICs).

FIG. 7 is a block diagram depicting another exemplary embodiment of asatellite navigation data distribution system 700 constructed inaccordance with the invention. Elements in FIG. 7 that are the same orsimilar to elements in FIG. 1 are designated with identical referencenumerals and are described in detail above. In the present embodiment,the system 700 further comprises a network management system 702 incommunication with the reference stations 102, the hubs 108 and 110, theserver 116, and the reference station 114. The reference stations 102,the hubs 108 and 110, the server 116, and the reference station 114 aregenerally referred to as “network elements.” Each of the referencestations 102, the hubs 108 and 110, the server 116, and the referencestation 114 includes a management agent 704. The network managementsystem 702 includes an interface to a human network manager. Forexample, the network management system 702 may include a graphical userinterface (GUI) 706 and/or an Internet or “web” interface 708 throughwhich a network manager may manage the network elements in the system700. Each management agent 704 provides an interface between the networkmanagement system 702 and its respective network element.

As described above, in operation, each of the network elements generallyprocesses satellite navigation data. For example, each of the referencestations 102 obtains satellite navigation data and distributes thecollected satellite navigation data to the hubs 108 and 110. Each of thehubs 108 and 110 concentrates the collected satellite navigation data toremove redundant information and relays the satellite navigation data tothe server 116. The server 116 may also concentrate the collectedsatellite navigation data and decodes the satellite navigation data, anddistributes the decoded data in response to requests. Each of thenetwork elements maintains a set of status variables associated with itsrespective processing of satellite navigation data.

In one embodiment, the status variables maintained by each networkelement relate to the integrity of the satellite navigation data, aswell as the integrity of the respective network element. For example,status variables related to the integrity of the satellite navigationdata include variables associated with: the number of satellitesrepresented by the satellite navigation data; ephemeris data (e.g.,time-of-ephemeris (TOE), time-of-week (TOW) values); almanac data;atmospheric correction data; clock correction data (e.g., UTC values);satellite health, and like type satellite navigation data parametersknown in the art. Status variables related to the integrity of a networkelement include variables associated with; the link status to othernetwork element(s) with which the network element communicates; statusof various devices employed by the network element (e.g., GPS receivers,processors, disk drives, and the like); status of software processesbeing executed by the network element; and the like.

Each of the management agents 704 monitors the states of the statusvariables maintained by its respective network element. Each of themanagement agents 704 is configured to compare the states of the statusvariables maintained by its respective network element to thresholds andgenerate alarms and notifications if the state of any status variableexceeds a threshold. Each of the management agents 704 may monitor eachof the status variables in accordance with various polling intervals.The alarms and notifications are sent to the network management system702, where they may be displayed to a network manager via the GUIinterface 706 and/or the web interface 708. In addition, the networkmanagement system 702 may forward information related to the alarms andnotifications to a remote terminal 710 via a communication network 712.For example, the network management system 702 may send e-mail messagesto a network manager, which may be read using various devices, such as acomputer, personal digital assistant (PDA), cellular telephone, and thelike. The network management system 702 may send short message service(SMS) text messages to a network manager, which may be read usingsimilar type devices.

For example, the management agent 704 in the hub 108 may monitor thestate of a status variable associated with TOE data. A threshold valuefor TOE data may be set that dictates an acceptable age for a TOE valueof a satellite navigation data stream. Any TOE value that exceeds thethreshold age is deemed to be too old. If the TOE of one or moresatellite navigation data streams exceeds a threshold value, themanagement agent 704 in the hub 108 generates an alarm, which is sent tothe network management system 702. The network management system 702displays the TOE alarm and/or forwards information related to the TOEalarm to a remote terminal. In another example, the management agent 704in the server 116 may monitor the state of a status variable associatedwith a disk drive that is used to cache satellite navigation data. Athreshold value for disk drive may be set that dictates an acceptableamount of free storage space. If the threshold is exceeded, the diskdrive is deemed to have low disk space and the management agent 704 inthe server 116 generates an alarm, which is sent to the networkmanagement system 702. The network management system 702 displays thelow disk space alarm and/or forwards information related to the low diskspace alarm to a remote terminal.

Each of the management agents 704 is further configured to receiverequest messages from the network management system 702 and send replymessages to the network management system 702. For example, the networkmanagement system 702 may request the state of a given variablemaintained by a network element. The management agent 704 for thenetwork element then responds to the network management system 702 withthe current state of the variable. In another example, the networkmanagement system 702 may set a value of a threshold for a givenvariable maintained by a network element. The management agent 704 forthe network element then acknowledges the change in the threshold value.

In one embodiment, the satellite distribution system 700 may be managedin accordance with the simple network management protocol (SNMP). Thenetwork management system 702 exchanges information with each managementagent 704 using a small set of commands. Notably, the network managementsystem 702 may issue a SET command to set a value of a status variableor set a value of a threshold associated with a status variable. Thenetwork management system 702 may issue a GET command to retrieve avalue of a status variable. Each management agent 704 may issue a TRAPmessage to spontaneously inform the network management system 702 of animportant event (e.g., an alarm). Each management agent 704, as well asthe network management system 702, maintains a management informationbase (MIB) containing the managed status variables. A numeric tag orobject identifier (OID) is used to distinguish each status variableuniquely in the MIB and in SNMP messages. The SNMP protocol is wellknown in the art.

FIG. 8 is a data flow diagram depicting an exemplary embodiment of amethod 800 for managing a network element 802 in the satellitenavigation data distribution system 700 in accordance with theinvention. In the present embodiment, the management agent 704 of thenetwork element 802 comprises a monitoring agent 804, a proxy agent 806,and an SNMP agent 808. For example, the network element 802 may comprisea computer 600 of FIG. 6, and the management agent 704 may beimplemented using software stored in the memory 603. Those skilled inthe art will appreciate that the management agent 704 may also beimplemented as hardware or a combination of hardware and software.

The monitoring agent 804 performs the actual monitoring of statusvariables in a storage location 812 and the generation of alarms inaccordance with established thresholds in a storage location 811. Thestorage locations 812 and 822 may be in the memory 603 of the computer600. The monitoring agent 804 periodically determines state data 824 forthe status variables in the storage location 812 in accordance withpolling intervals. Notably, the monitoring agent 804 may determine thestate of individual status variables using different polling intervals.The monitoring agent 804 compares the state data with threshold data 822obtained from the storage location 811. If the state of any statusvariable exceeds its respective threshold value, the monitoring agent804 generates an alarm. In response to one or more alarms, themonitoring agent 804 sends alarm data 820 to the proxy agent 806.

The proxy agent 806 writes a TRAP message 818 having the alarm data 820to a file in a storage location 810. The storage location 810 may be inthe memory 603 of the computer 600. The SNMP agent 808 reads the TRAPmessage 818 from a file in the storage location 810. The SNMP agent 808sends SNMP message data 814 having the TRAP message 818 to the networkmanagement system 702. In this manner, an alarm generated at the networkelement 802 is reported to the network management system.

FIG. 9 is a data flow diagram depicting another exemplary embodiment ofa method 900 for managing the network element 802 in accordance with theinvention. The network management system 702 sends SNMP request data 902to the SNMP agent 808. The SNMP request data 902 may comprise a GET or aSET message. The SNMP agent 808 writes a request message 904 to a filein the storage location 810. The proxy agent 806 reads the requestmessage 904 from a file in the storage location 810. The proxy agent 806forwards the request message 904 to the monitoring agent 804. Themonitoring agent 804 processes the request message 904 in accordancewith the type of message. For example, if the request message 904 isrequesting state data for one or more status variables, the monitoringagent 804 reads state data 912 from the storage location 812. If therequest message 904 is requesting a change to a threshold, themonitoring agent 804 writes threshold data 910 to the storage location811. The monitoring agent 804 may then respond to the network managementsystem 702 in the form of an acknowledgment message. The acknowledgmentmessage may be sent to the network management system 702 in a similarmanner as described above in the method 800 for an alarm message.

Although the methods and apparatus of the invention have been describedwith reference to GPS satellites, it will be appreciated that theteachings are equally applicable to positioning systems that utilizepseudolites or a combination of satellites and pseudolites. Pseudolitesare ground-based transmitters that broadcast a PN code (similar to theGPS signal) that may be modulated on an L-band carrier signal, generallysynchronized with GPS time. The term “satellite”, as used herein, isintended to include pseudolites or equivalents of pseudolites, and theterm “GPS signals”, as used herein, is intended to include GPS-likesignals from pseudolites or equivalents of pseudolites.

Moreover, in the preceding discussion, the invention has been describedwith reference to application upon the United States Global PositioningSystem (GPS). It should be evident, however, that these methods areequally applicable to similar satellite systems, and in particular, theRussian Glonass system and the European Galileo system. The term “GPS”used herein includes such alternative satellite positioning systems,including the Russian Glonass system and the European Galileo system.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A network element in a satellite navigation data distribution system,comprising: a processor for processing satellite navigation data; amemory for maintaining status variables associated with said processingof said satellite navigation data; and a management agent for monitoringstates of said status variables and communicating with a networkmanagement system to exchange information related to said states of saidstatus variables.
 2. The network element of claim 1, wherein said statusvariables include at least one of: a variable associated with a numberof satellites represented by said satellite navigation data; at leastone variable associated with ephemeris data in said satellite navigationdata; at least one variable associated with almanac data in saidsatellite navigation data; at least one variable associated withatmospheric correction data in said satellite navigation data; and atleast one variable associated with clock correction data in saidsatellite navigation data.
 3. The network element of claim 1, whereinsaid management agent is configured to communicate using a simplenetwork management protocol (SNMP).
 4. The network of claim 3, whereinsaid first management agent comprises: a monitoring agent for comparingsaid states of said status variables to thresholds and selectivelygenerating alarm messages in response to said comparison; an SNMP agentfor interfacing with said network management system; and a proxy agentfor providing an interface between said SNMP agent and said monitoringagent.
 5. A method of managing a network element in a satellitenavigation data distribution system, comprising: maintaining statusvariables at said network element in response to satellite navigationdata processed by said network element; monitoring states of said statusvariables; and communicating information related to said states of saidstatus variables between said network element and a network managementsystem.
 6. The method of claim 5, wherein said step of communicatingcomprises: receiving request messages at said network element from saidnetwork management system; and sending reply messages from said networkelement to said network management system in response to said requestmessages.
 7. The method of claim 5, wherein said step of monitoringcomprises: comparing said states of said variables with associatedthresholds; and generating at least one alarm message in response tosaid comparison.
 8. The method of claim 7, wherein said step ofcommunicating comprises: sending said at least one alarm message fromsaid network element to said network management system.
 9. The method ofclaim 5, wherein said status variables include at least one of: avariable associated with a number of satellites represented by saidsatellite navigation data; at least one variable associated withephemeris data in said satellite navigation data; at least one variableassociated with almanac data in said satellite navigation data; at leastone variable associated with atmospheric correction data in saidsatellite navigation data; and at least one variable associated withclock correction data in said satellite navigation data.
 10. The methodof claim 5, wherein said information is communicated using a simplenetwork management protocol (SNMP).
 11. A system for distributingsatellite navigation data, comprising: a plurality of reference stationsfor processing satellite signals to obtain satellite navigation data; aserver for obtaining said satellite navigation data and maintainingfirst status variables, said server including a first management agentfor monitoring states of said first status variables; and a networkmanagement system in communication with said first management agent forexchanging messages associated with said states of said first statusvariables.
 12. The system of claim 11, wherein each reference station ofsaid plurality of reference stations includes: a second management agentfor monitoring states of second status variables maintained by saidreference station; wherein said network management system is incommunication with said second management agent for exchanging messagesassociated with said states of said second status variables.
 13. Thesystem of claim 11, further comprising: a hub configured to provide saidsatellite navigation data from said plurality of reference stations tosaid server and to maintain third status variables, said hub including athird management agent for monitoring states of said third statusvariables; wherein said network management system is in communicationwith said third management agent for exchanging messages associated withsaid states of said third status variables.
 14. The system of claim 11,wherein said first status variables include at least one of: a variableassociated with a number of satellites represented by said satellitenavigation data; at least one variable associated with ephemeris data insaid satellite navigation data; at least one variable associated withalmanac data in said satellite navigation data; at least one variableassociated with atmospheric correction data in said satellite navigationdata; and at least one variable associated with clock correction data insaid satellite navigation data.
 15. The system of claim 11, wherein saidnetwork management system is configured to exchange said messagesassociated with said states of said first status variables using asimple network management protocol (SNMP).
 16. The system of claim 15,wherein said first management agent comprises: a monitoring agent forcomparing said states of said first status variables to thresholds andselectively generating alarm messages; an SNMP agent for interfacingwith said network management system; and a proxy agent for providing aninterface between said SNMP agent and said monitoring agent. 17.Apparatus for managing a network element in a satellite navigation datadistribution system, comprising: means for maintaining status variablesat said network element in response to satellite navigation dataprocessed by said network element; means for monitoring states of saidstatus variables; and means for communicating information related tosaid states of said status variables between said network element and anetwork management system.
 18. The apparatus of claim 17, wherein saidmeans for communicating comprises: means for receiving request messagesat said network element from said network management system; and meansfor sending reply messages from said network element to said networkmanagement system in response to said request messages.
 19. Theapparatus of claim 17, wherein said means for monitoring comprises:means for comparing said states of said variables with associatedthresholds; and means for generating at least one alarm message inresponse to said comparison.
 20. The apparatus of claim 19, wherein saidmeans for communicating comprises: means for sending said at least onealarm message from said network element to said network managementsystem.